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robbst
2007-Nov-08, 05:47 AM
My own understanding - the Universe is approx 13 billion years old.
Hubble has detected light from as far back as 10 + billion years.

I heard, from what seemed a reliable source, that we can only see 3 - 4% of the Universe because of its vast size.

Am I missing something here - is the Universe substantially bigger than what we can see (back to 10 billion years) ?

mystified

Neverfly
2007-Nov-08, 06:03 AM
According to Space.com (http://www.space.com/scienceastronomy/mystery_monday_040524.html) The universe is at least 156 billion light-years wide.

astromark
2007-Nov-08, 06:48 AM
Wide deep and high... Yes we can not see it all as its distance and velocity prohibit the light image ever reaching us... The rate of expansion suggests that bigger is best guess. 156 billion light years is a long, long way. Its also a long, long time. Do not let these numbers destruct you from the base fact that we can see 13.7 billion light years.

Spaceman Spiff
2007-Nov-08, 03:00 PM
A lot of misconceptions floating about here, which lead to further confusion. Don't worry, this is difficult even for the experts.

Read this (http://en.wikipedia.org/wiki/Observable_universe), which lays to rest several misconceptions and provides good explanations, and related articles linked from that site. You should also visit here (http://www.mso.anu.edu.au/%7Echarley/papers/LineweaverDavisSciAm.pdf), here (http://www.atlasoftheuniverse.com/redshift.html) and here (http://www.atlasoftheuniverse.com/bigbang.html), and then maybe you'll be ready for the more advanced 'course' (http://www.astro.ucla.edu/%7Ewright/cosmolog.htm) (especially the FAQ and tutorial pages) :).

It's a journey well worth taking, as long as you don't expect to arrive at a "final destination" of understanding.

astromark
2007-Nov-08, 06:24 PM
Spaceman Spiff... To much information only confuses the question. Final Destination is in its self very confusing. Understanding is gained with information and in this you have provided all that the OP could ever want to know.. But to take it all in is a big ask. Some of the facts presented are not. As yet some explanation is still just theory. We do not know it all.

Neverfly
2007-Nov-08, 07:12 PM
Spaceman Spiff... To much information only confuses the question. Final Destination is in its self very confusing. Understanding is gained with information and in this you have provided all that the OP could ever want to know.. But to take it all in is a big ask. Some of the facts presented are not. As yet some explanation is still just theory. We do not know it all.

I agree. And thanks to Spaceman Spiff for laying it out in an easy to follow way. I've learned quite a few new things now too:p

A good introduction to a complicated answer leaves you feeling thirsty for more. Thanks for the links;)

speedfreek
2007-Nov-08, 08:59 PM
Yes, so many misconceptions surround this subject, that it is sometimes difficult to find the correct answer - i.e. the current mainstream view.

156 billion light years is incorrect.
13.7 billion light years is also incorrect!

The current mainstream view is that the universe is 13.7 billion years old. That does not however mean that our observable universe is 13.7 billion light years in radius and this is due to the metric expansion of space.

We see the most distant objects as they were when they emitted their light, something over 13 billion years ago. We see their apparent distance, when measured using their angular diameter, as being relatively close - only 2 or 3 billion light years away. But they are so dim and redshifted due to their light having travelled through space that has been expanding, that it is estimated that their light took over 13 billion years to reach us due to the space in between them and this point in space expanding, and we now estimate them to be anything up to 46 billion light years away.

So our observable universe has a radius of 46 billion light years now, the most distant objects look very dim and highly redshifted but their angular size shows they were only a few billion light years away when they emitted the light we now see. The 13.7 billion light years figure is irrelevant when used as a measure of distance in this context.

A recent study has established a lower bound on the size of the whole universe as 78 billion light years in diameter, which is actually smaller than our observable universe of 46 billion light years radius - the 156 billion light year figure quoted is an erroneous doubling of that 78 billion light year figure, based on the misconception that it was a radius and that it should not be smaller than our observable universe. In fact, that figure only establishes a lower bound because that was a far as the study looked! The whole universe may indeed be magnitudes larger than our observable portion of it.

That study then went on to suggest that if the whole universe were smaller than our observable universe then light might have had time to circumnavigate it and we might be observing the same region in space when looking in different directions. They performed a matching circle analysis in order to see if that were the case, but found no matches across a distance of 78 billion light years. As they were looking at data from WMAP, this 78 billion light year figure can be considered to be either a diameter or a circumference, depending on how you interpret the WMAP data (when looking around a spherical model from the inside, if you pan from left to right, are you measuring around the other edge or across the middle?!). :)

When talking about the observable universe as opposed to the whole universe, it is important to remember that if we theorise that our observable universe was, just after the beginning, only the size of a grapefruit, then someone residing in another part of the whole universe, wholly outside of our observable universe, when using the same theory, is actually looking at a totally different grapefruit to our own. If they live in a part of the universe that cannot see any part of our observable universe now, the same has been true all the way back in time towards the beginning when the expansion was at its fastest.

Even when we talk of epochs just after the beginning, when our observable universe was very small, the whole universe might already have been very large.

Spaceman Spiff
2007-Nov-09, 01:12 AM
Speedfreek - thanks for taking the time to summarize some of the major points made in the links that I provided which pertain directly to the original question.

Astromark - no, we do not know it all. As a scientist I don't think that such a goal is even attainable, and I do not expend any energy entertaining such thoughts. However, we have a working - and so far very successful - scientific theory ("just a theory" it is not) of how the universe behaves on the very large scales as a function of time due to its contents of matter and energy within the framework of yet another very successful scientific theory called "General Relativity". While even the qualitative aspects of the Big Bang theory are difficult for most to grasp (and even the experts occasionally make conceptual errors), that is not the fault of the theory.

You will not get the answers you seek in a sound bite, or even in the single excellent post by Speedfreek. That is why I supplied those starting place websites for you and others to read on their own time.

Astromark: If I've in any way misconstrued the meanings in any of your statements, my apologies.

astromark
2007-Nov-09, 05:43 AM
No Spaceman Spiff you have not got me wrong... and I agree with you. That is what I said. Only by reading more and researching this topic can more be learned.

Tzarkoth
2007-Nov-09, 04:32 PM
Thank you Spaceman Spiff, very informative links.

13.7 Billion years ago, would not all matter have been gravitationally bound. How then is it we get to our present model of the Universe where Galaxies etc are moving away from each other yet not becoming bigger themselves?

I think I missed the part where they explained that.

Spaceman Spiff
2007-Nov-09, 05:07 PM
Thank you Spaceman Spiff, very informative links.

13.7 Billion years ago, would not all matter have been gravitationally bound. How then is it we get to our present model of the Universe where Galaxies etc are moving away from each other yet not becoming bigger themselves?

I think I missed the part where they explained that.

The Standard Big Bang Theory does not address "what banged" or the mechanism behind the expansion. Other theoretical models have been invoked to do this - namely some form of "Inflation" (Alan Guth, and others). The standard BBT describes and explains the evolution of matter and energy over the largest of distance scales in a universe expanding in time. As matter and energy cooled during the expansion, local pockets of over dense regions (originally statistical quantum fluctuations) collapsed gravitationally (i.e., the local gravitational field effectively prevented the continued dilution of the local matter/energy, and in fact reversed it to collapse) to form dense halos, which themselves collapsed to form super star clusters which merged to form galaxies - and together formed galaxy clusters and groups. On the scales of galaxy clusters and smaller, the local gravitational field (and other forces - for example, electrical forces between molecules for you and I) completely dominates over the expansion of space with time.

I have purposefully left out the role of dark matter, and the above is just the briefest of explanations of how we understand this to have happened.

orochi
2007-Nov-13, 04:44 AM
According to Janna Levin, in an email she sent me, the universe is actually only 100 MILLION light years! Everything else is an illusion!

It could be a small universe after all!

astromark
2007-Nov-13, 05:10 AM
Oh ya... its very small.

Its the only one we know of. How can we call it small ?

Orochi., Ignore the likes of J Levin... That is not science.

Read what Spaceman Spiff has posted.. That is.

Neverfly
2007-Nov-13, 06:15 AM
I second the motion.